Chronic Histological Outcomes of Indirect Traumatic Optic Neuropathy in Adolescent Mice: Persistent Degeneration and Temporally Regulated Glial Responses

Visual Impairment in Pre-Clinical Models of Mild Traumatic Brain Injury

Impairment in visual function is common after traumatic brain injury (TBI) in the clinical setting, a phenomenon that translates to pre-clinical animal models as well. In Morris et al. (2021), we reported histological changes following weight-drop-induced TBI in a rodent model including retinal ganglion cell (RGC) loss, decreased electroretinogram (ERG) evoked potential, optic nerve diameter reduction, induced inflammation and gliosis, and loss of myelin accompanied by markedly impaired visual acuity. In this review, we will describe several pre-clinical TBI models that result in injuries to the visual system, indicating that visual function may be impaired following brain injury induced by a number of different injury modalities. This underscores the importance of understanding the role of the visual system and the potential detrimental sequelae to this sensory modality post-TBI. Given that most commonly employed behavioral tests such as the Elevated Plus Maze and Morris Water Maze rely on an intact visual system, interpretation of functional deficits in diffuse models may be confounded by off- target effects on the visual system.

Complement propagates visual system pathology following traumatic brain injury

BACKGROUND

Traumatic brain injury (TBI) is associated with the development of visual system disorders. Visual deficits can present with delay and worsen over time, and may be associated with an ongoing neuroinflammatory response that is known to occur after TBI. Complement system activation is strongly associated with the neuroinflammatory response after TBI, but whether it contributes to vision loss after TBI is unexplored.

METHODS

Acute and chronic neuroinflammatory changes within the dorsal lateral geniculate nucleus (dLGN) and retina were investigated subsequent to a moderate to severe murine unilateral controlled cortical impact. Neuroinflammatory and histopathological outcomes were interpreted in the context of behavioral and visual function data. To investigate the role of complement, cohorts were treated after TBI with the complement inhibitor, CR2-Crry.

RESULTS

At 3 days after TBI, complement component C3 was deposited on retinogeniculate synapses in the dLGN both ipsilateral and contralateral to the lesion, which was reduced in CR2-Crry treated animals. This was associated with microglia morphological changes in both the ipsilateral and contralateral dLGN, with a less ramified phenotype in vehicle compared to CR2-Crry treated animals. Microglia in vehicle treated animals also had a greater internalized VGlut2 + synaptic volume after TBI compared to CR2-Crry treated animals. Microglia morphological changes seen acutely persisted for at least 49 days after injury. Complement inhibition also reduced microglial synaptic internalization in the contralateral dLGN and increased the association between VGLUT2 and PSD95 puncta, indicating preservation of intact synapses. Unexpectedly, there were no changes in the thickness of the inner retina, retinal nerve fiber layer or retinal ganglion layer. Neuropathological changes in the dLGN were accompanied by reduced visual acuity at subacute and chronic time points after TBI, with improvement seen in CR2-Crry treated animals.

CONCLUSION

TBI induces complement activation within the dLGN and promotes microglial activation and synaptic internalization. Complement inhibition after TBI in a clinically relevant paradigm reduces complement activation, maintains a more surveillance-like microglia phenotype, and preserves synaptic density within the dLGN. Together, the data indicate that complement plays a key role in the development of visual deficits after TBI via complement-dependent microglial phagocytosis of synapses within the dLGN.

Complement propagates visual system pathology following traumatic brain injury

Background

Traumatic brain injury (TBI) is associated with the development of visual system disorders. Visual deficits can present with delay and worsen over time, and may be associated with an ongoing neuroinflammatory response that is known to occur after TBI. Complement activation is strongly associated with the neuroinflammatory response after TBI, but whether it contributes to vision loss after TBI is unexplored.

Methods

Acute and chronic neuroinflammatory changes within the dorsal lateral geniculate nucleus (dLGN) and retina were investigated subsequent to murine controlled unilateral cortical impact. Neuroinflammatory and histopathological data were interpreted in the context of behavioral and visual function data. To investigate the role of complement, cohorts were treated after TBI with the complement inhibitor, CR2-Crry.

Results

At 3 days after TBI, complement C3 was deposited on retinogeniculate synapses in the dLGN both ipsilateral and contralateral to the lesion, which was reduced in CR2-Crry treated animals. This was associated with microglia morphological changes in both the ipsilateral and contralateral dLGN, with a more amoeboid phenotype in vehicle compared to CR2-Crry treated animals. Microglia in vehicle treated animals also had a greater internalized VGlut2+ synaptic volume after TBI compared to CR2-Crry treated animals. Microglia morphological changes seen acutely persisted for at least 49 days after injury. Complement inhibition also reduced microglial synaptic internalization in the contralateral dLGN and increased the association between VGLUT2 and PSD95 puncta, indicating preservation of intact synapses. Unexpectedly, there were no changes in the thickness of the inner retina, retinal nerve fiber layer or retinal ganglion layer. Pathologies were accompanied by reduced visual acuity at subacute and chronic time points after TBI, with improvement seen in CR2-Crry treated animals.

Conclusion

TBI induces complement activation within the dLGN and promotes microglial activation and synaptic internalization. Complement inhibition after TBI in a clinically relevant paradigm reduces complement activation, maintains a more surveillance-like microglia phenotype, and preserves synaptic density within the dLGN. Together, the data indicate that complement plays a key role in the development of visual deficits after TBI via complement-dependent microglial phagocytosis of synapses within the dLGN.

Model matters: Differential outcomes in traumatic optic neuropathy pathophysiology between blunt and blast-wave mediated head injuries

Over 3 million people in the United States live with long-term disability because of a traumatic brain injury (TBI). The purpose of this study was to characterize and compare two different animal models of TBI (blunt head trauma and blast TBI) to determine common and divergent characteristics of these models. With recent literature reviews noting the prevalence of visual system injury in animal models of TBI, coupled with clinical estimates of 50-75% of all TBI cases, we decided to assess commonalities, if they existed, through visual system injury. A unilateral (left directed) blast and repeat blast model injury with coup-contra-coup injury patterns were compared to a midline blunt injury. Injuries were induced in adult male mice to observe and quantify visual deficits. Retinal ganglion cell loss and axonal degeneration in the optic tract, superior colliculus, and lateral geniculate nuclei were examined to trace injury outcomes throughout major vision-associated areas. Optokinetic response, immunohistochemistry, and western blots were analyzed. Where a single blunt injury produces significant visual deficits a single blast injury appears to have less severe visual consequences. Visual deficits after repeat blasts are similar to a single blast. Single blast injury induces contralateral damage to the right optic chiasm and tract whereas bilateral injury follows a single blunt TBI. Repeat blast injuries are required to see degeneration patterns in downstream regions similar to the damage seen in a single blunt injury. This finding is further supported by amyloid precursor protein (APP) staining in injured cohorts. Blunt injured groups present with staining 1.2 mm ahead of the optic nerve, indicating axonal breakage closer to the optic chiasm. In blast groups, APP was identifiable in a bilateral pattern only in the geniculate nucleus. Evidence for unilateral neuronal degeneration in brain tissue with bilateral axonal ruptures are pivotal discoveries in this model differentiation. Analysis of the two injury models suggests that there is a significant difference in the histological outcomes dependent on injury type, though visual system injury is likely present in more cases than are currently diagnosed clinically.

Brief oxygen exposure after traumatic brain injury speeds recovery and promotes adaptive chronic endoplasmic reticulum stress responses

Traumatic brain injury (TBI) is a major public health concern particularly in adolescents who have a higher mortality and incidence of visual pathway injury compared to adult patients. Likewise, we have found disparities between adult and adolescent TBI outcomes in rodents. Most interestingly, adolescents suffer a prolonged apneic period immediately post injury leading to higher mortality; so, we implemented a brief oxygen exposure paradigm to circumvent this increased mortality. Adolescent male mice experienced a closed-head weight-drop TBI then were exposed to 100% O ₂ until normal breathing returned or recovered in room air. We followed mice for 7- and 30-days and assessed their optokinetic response; retinal ganglion cell loss; axonal degeneration; glial reactivity; and retinal ER stress protein levels. O ₂ reduced adolescent mortality by 40%, improved post-injury visual acuity, and reduced axonal degeneration and gliosis in optic projection regions. ER stress protein expression was altered in injured mice, and mice given O ₂ utilized different ER-stress pathways in a time dependent manner. Finally, O ₂ exposure may be mediating these ER stress responses through regulation of the redox-sensitive ER folding protein ERO1α, which has been linked to a reduction in the toxic effects of free radicals in other animal models of ER stress.

Oxidative stress in the brain and retina after traumatic injury

The brain and the retina share many physiological similarities, which allows the retina to serve as a model of CNS disease and disorder. In instances of trauma, the eye can even indicate damage to the brain via abnormalities observed such as irregularities in pupillary reflexes in suspected traumatic brain injury (TBI) patients. Elevation of reactive oxygen species (ROS) has been observed in neurodegenerative disorders and in both traumatic optic neuropathy (TON) and in TBI. In a healthy system, ROS play a pivotal role in cellular communication, but in neurodegenerative diseases and post-trauma instances, ROS elevation can exacerbate neurodegeneration in both the brain and the retina. Increased ROS can overwhelm the inherent antioxidant systems which are regulated via mitochondrial processes. The overabundance of ROS can lead to protein, DNA, and other forms of cellular damage which ultimately result in apoptosis. Even though elevated ROS have been observed to be a major cause in the neurodegeneration observed after TON and TBI, many antioxidants therapeutic strategies fail. In order to understand why these therapeutic approaches fail further research into the direct injury cascades must be conducted. Additional therapeutic approaches such as therapeutics capable of anti-inflammatory properties and suppression of other neurodegenerative processes may be needed for the treatment of TON, TBI, and neurodegenerative diseases.